[en] In the present thesis, it is argued that (1) the Accession Treaties have been used in accordance with their nature and proclaimed objective: they only brought about limited changes to primary law, proper to the needs of accession and have not introduced any fundamental changes. The numerous and still growing arrangements that depart from the principle of the application of the acquis in toto on accession do not alter this conclusion. (2) The evolution, especially from the 2004 Accession Treaty onwards and predictable for the future Accession Treaties (e.g. with Turkey), shows a tendency of diversification of that legal instrument by (a) adding new and/or reinforced elements of conditionality, protracted from the pre-accession phase to the membership phase, devising new mechanisms of conditionality and control (general and specific safeguard clauses, monitoring and verification mechanisms, membership postponement clause) and thus (b) contributing to a further differentiation in two respects - as among the Member States and with regard to the core acquis. Such differentiation exists already on the basis of the constitutive treaties (“in-built constitutive treaties induced differentiation”) and is accentuated by the Accession Treaties and their transitional arrangements (“Accession Treaties induced differentiation”). Questions of differentiation acquired another dimension with the introduction of the citizenship of the EU. (3) Finally, negotiations with certain candidate countries will show whether additional innovations are to be expected: a) whether future instruments of accession would be used in order to increase the existing level of differentiation (and protract the pre-accession phase logic well into the membership phase) with the conditionality becoming the most important element of the relations within an enlarged EU and thus paradoxically negating the nature of the integration itself, b) whether they will perhaps be used to bring about more important modifications to the treaties, or c) whether they will go as far as to provide a legal basis for permanent derogations with regard to certain new Member States (as explicitly envisaged, for instance, in the negotiating framework for Turkey).